Tag Archives: skeletal muscles

Vicious Cycle – Aging and Declining Blood Sugar Control

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Blog 18 – Age changes make it difficult to control blood sugar within a normal range.  Persistently high sugar levels create many problems.  The most significant is an acceleration of  aging that further promotes poor blood sugar control and hence high blood sugar. 

Hence, a vicious cycle is created that leads to preliminary stages of diabetes.  The origins of this cycle and effective means to disrupt it are presented in this blog and the next.

Introduction

Type 2 Diabetes (T2D) is the 7th leading cause of death in the elderly.  It is an insidious disease because there are no obvious early warning symptoms.  Moreover, T2D accelerates normal aging, increases the risk for heart disease and damages small blood vessels in multiple organs causing blindness, gangrene, reduced kidney function, impotence and neuropathy.  Fortunately, it is a potentially avoidable disease.

Prevention of T2D begins with assessment of fasting levels of blood sugar (clinically termed glucose).  A glucose concentration equal to or greater than 110 mg/100 ml of plasma after a 12-15 hour fast, obtained from repeat testing over 12 months, indicates an early but highly reversible stage of diabetes, termed prediabetes.  Other tests, HbA1c and glucose tolerance test (see Table 1 for details) are required to complement the fasting glucose values and could indicate progression to T2D.

Based on the most recent estimates by the CDC (2019), 26.4 million individuals over 65 have prediabetes based on results from one or more tests noted above.  However, this is an underestimation since many elderly avoid testing and thus remain uninformed on their risk for T2D.  

With a diagnosis of prediabetes, clinicians advise diet modification, weight loss and exercise (https://www.cdc.gov/diabetes/data/statistics-report/index.html).  While these strategies are effective if strictly followed, it is equally important for the elderly to understand the underlying reasons for the prediabetes in the first place and to have the option to minimize them.

The Influence of Aging on Blood Sugar Control

T2D is an age-related disease because aging influences blood sugar in two significant ways. 

    *One major age-related change is the well-documented decline in skeletal muscle mass. 

    *The second major change is the more recently documented age-related increase in the stress  hormone, cortisol.  Specifically, both resting and activated levels of cortisol increase with age and the response to stress is prolonged. 

Together, these age-allied modifications promote and sustain higher than normal levels of blood glucose, and thus create the prediabetic state.  Additionally, persistently high levels of glucose accelerate the deteriorative effects of aging, further disrupting glucose regulation.

Factor 1 – Age-Related Decline in Skeletal Muscle Mass

Several hormones and associated factors regulate blood glucose.  The most significant and most studied is insulin, a pancreatic hormone.  Following ingestion of food, metabolic processes liberate breakdown products such as glucose, amino acids and fatty acids into the blood.  Insulin, whose secretion is stimulated by rising glucose levels, facilitates the movement of these energy-rich nutrients especially glucose into the skeletal muscles for immediate use and into the fat and liver for storage. 

The problem for the elderly is this.  Skeletal muscle mass, comprising a significant portion (~40%) of total body mass, slowly declines with age.  It is the biggest consumer of available glucose whose delivery is assured by insulin.  However, if food consumption remains constant with age as it usually does, the same amount of glucose is still available BUT the end user e.g. the skeletal muscles, are reduced in size so less glucose is taken up.  Therefore, more glucose remains in the circulation and eventually is stored in the liver and fat.

The point is that with loss of muscle mass, circulating glucose remains higher for longer periods of time, supporting a prediabetic state.

Minimize Muscle Loss with Progressive Resistance Exercise

To keep blood glucose normal, muscle mass must remain constant.  Sadly, the loss of muscle mass with age is not obvious to most elderly.  This is because disappearing muscle tissue is replaced with fat deposits.  The only noticeable changes are a decline in muscle strength and associated unsteady balance.  A wealth of clinical trial data indicates that a program of progressive resistance exercise slows loss of muscle mass (and strength too).  It also improves insulin function called insulin sensitivity.  Insight 3, Progressive Resistance Exercise and Protein Supplements details these programs which may include free weights, resistance bands, weight machines and/or calisthenics.

An important corollary to a program of progressive resistance exercise is the recommendation to increase consumption of  proteins.  The building blocks of proteins are amino acids.  Amino acids stimulate muscle formation and are also incorporated into essential muscle proteins.  Quality proteins such as whey, soy and casein contain these amino acids. 

According to the government’s Recommended Daily Allowance (RDA), adults need to consume 56 grams of  protein each day to maintain general health.  In contrast, biogerontologists whose focus is the health span of the elderly, found the RDA to be inadequate for the elderly.  Consequently, the new, science-based recommendation for the elderly is twice the RDA.  To optimize muscle size and function, it is reasonable to consume quality proteins (whey, soy, casein containing essential amino acids) with meals and about 2 hours after a workout of resistance exercises.

Two Opposing Sides to Glucose

Glucose is an excellent source of energy for all cells in the body.  In contrast, it has a destructive side.  Glucose is a notorious oxidizer.  This means that glucose has the potential to  indiscriminately attack and damage nearby proteins, fats and DNA in tissues and cells.  Such oxidative modifications, if not repaired, contribute to reduced function and inflammation, hallmarks of aging.  Oxidative damage across the board makes it infinitely harder to keep glucose within an acceptable physiological range.

Vicious Cycle

Conclusions

It is incredibly important to maintain adequate control of  blood glucose.  In particular, glucose needs to be around long enough to provide a ready supply of energy to our muscles, but not long enough to exert oxidative damage.  Unfortunately, glucose is higher from longer periods of time in the elderly. This is because the muscles, the major consumer of glucose, diminish in size with age.  A program of progressive resistance exercise maintains muscle mass. However, absent this strategy, there is a risk of oxidative destruction to tissues and cells and a platform for prediabetes.

The second obstacle to  control of normal blood sugar is the age-related change in the stress response.  This is the topic of my next blog.

Table 1 – Tests Assessing Blood Sugar Control

TestMeasurementAbnormal Glucose Control  
Fasting GlucoseConcentration of glucose in the blood after
12-15 hour fast		 Equal to or greater than
110 milligrams/100 ml
HbA1cOxidation (glycation) of hemoglobin Equal to or greater than 5.7%
Glucose Tolerance TestGlucose levels following ingestion of 1 gram sugar/kg body wt. Glucose peak >140 mg/100 ml;
4-6 hours to return to near baseline
Select References:

Duchowny K et al., Muscle Weakness and Physical Disability in Older Americans: Longitudinal Findings from the U.S. Health and Retirement Study. Nutr Health Aging.  22(4): 501–507, 2018.

Gomes MJ et al., Skeletal muscle aging: influence of oxidative stress and physical exercise. Oncotarget 8: 20428-20440, 2017.  

Paddon-Jones, Rasmussen BB. Dietary protein recommendations and the prevention of sarcopenia.  Curr Opin Clin Nutr Metab Care. 12(1):  86–90, 2009.

Yau JW, Thor SM, Ramadas A. Nutritional Strategies in Prediabetes: A Scoping Review of Recent Evidence. Nutrients  12:  2990, 2020.

Zand A, Ibrhim K, Patham B. Prediabetes: Why Should We Care?  Methodist Debakey Cardiovasc J. 14 (4):  289-297, 2018.

Insight 2: Skeletal muscles, aging and consequences

Exercise, Individual controls aging, , , ,

Gradual loss of skeletal muscle mass and skeletal muscle strength is the most insidious and perilous age change of all

In my first blog, I explained that the trajectory of the aging process depends largely on an individual’s lifestyle choices.  It is, therefore, important that every older adult understand exactly what age changes can be expected and secondly, and most importantly, how to negate or minimize them.  This blog will describe one of the most troubling and debilitating effects of aging, loss of skeletal muscle mass (also referred to as muscle size) and loss of skeletal muscle strength.  I will follow this up with insight 3 (my next blog) on what to do about these changes.  Specifically, I will detail successful strategies of progressive resistance training plus consumption of quality protein.

Muscle mass and strength decline over time

It is well established that muscle mass and strength gradually decline over time.  A starting point, although variable and depending on the daily level of physical activity, is generally denoted at about 50-60 years of age but may start many years earlier.  Loss of muscle mass, termed sarcopenia, is about 1%/year whereas loss of muscle strength, designated dynapenia, is much greater, at about 3%/year.  Unfortunately, these changes go unnoticed especially in the case of declining muscle size since fat accumulation sneaks in to replaces muscle cells that either shrink or disappear.  This is illustrated in the MRI scans shown below.  Often muscle weakness is regrettably accepted as an inevitable and unalterable age change.

Just a brief note on the terms, sarcopenia and dynapenia.  A diagnosis of either sarcopenia (loss of muscle mass) or dynapenia (loss of muscle strength) indicates that a measureable quantity of decline in muscle structure and function has been determined.  Although not set in stone, there are defined numerical “cut-off” values (e.g. values for mass of arms/legs, grip strength, force of knee extension, speed of walking or rising from a chair)  associated with each term that have been established by professional medical research groups worldwide.  If a patient undergoes an assessment in which mass and strength are quantified and the numerical values fall below the established cut-off, the physician will make a diagnosis of either sarcopenia or dynapenia or both and propose appropriate therapy to prevent a worsening of these losses.  However, the goal for the older adult should be to continuously optimize muscle size and strength so that a designation of sarcopenia/dynapenia is never obtained or even considered.

Consequences of loss of muscle strength (dynapenia) – abundant, negative, and life-shortenin

Why be concerned about loss of muscle strength?  Because this change definitely leads to:

(1) increased physical disability; decreased quality of life,

(2) increased risk of falling,

(3) shorter lifespan. 

The most destructive change induced by dynapenia is the most obvious:  reduced leg, chest, back, shoulder and arm strength/power (speed) that slow and hinder performance in all daily activities from standing to walking to lifting to breathing.  As mobility and gait speed decline so does the total level of physical activity, further accelerating the decline in strength and power.  This inevitably leads to physical disabilities, loss of independence, and reduced quality of life.  Secondly, diminished skeletal muscle strength alters posture, a change which causes unsteady balance and an elevated risk of a fall.  Falls are menacing events with a high probability of a fracture,  hospitalization and lengthy recovery.  Additionally, weakened chest, back and shoulder muscles secondarily compromise the ability to augment the exchange of oxygen and carbon dioxide during stressful activities e.g. climbing stairs.  A reduction in gas exchange generally slows or halts the activity and reduces independence.  Dynapenia not only translates into poor physical performance and physical disabilities but, sadly, it has been statistically associated with increased mortality (premature death).

Consequences of loss of muscle mass (sarcopenia)

Why be concerned about loss of muscle mass?  Because this change definitely leads to:  

(1) weight gain,  

(2) elevated risk for Type 2 Diabetes 

(3) cold intolerance.

Weight gain occurs because skeletal muscles burn up a lot of calories just for maintenance.  The totality of muscle mass is huge and exceeds that of all other tissues combined.  Less muscle tissue means less calories consumed by muscles and more calories converted to fat storage and hence an associated weight gain.  In addition to the increased poundage, accumulated fat in the older adult locates, for as yet poorly understood reasons, to sites (abdomen or waist area; on top of major organs such as the heart) that encourage chronic low level inflammation, a major factor contributing to tissue damage.  Clearly an unwanted effect.  Secondly, muscles are one of the prime tissue targets that readily acquire ingested sugars, a process facilitated by insulin.  Less muscle, less uptake of sugar by this tissue and more sugar remaining to circulate.  Persistently elevated sugar levels augment the risk for Type 2 Diabetes and furthermore, promote spontaneous oxidative damage (a type of tissue damage) throughout the body, another unwanted effect that accelerates aging.  Finally, an often overlooked function of skeletal muscles is heat production in the form of shivering at low ambient temperatures.  Less muscle mass means less vigorous shivering and reduction in expected warmth.  This is experienced as cold intolerance which means that at low ambient temperatures, one needs to put on more outerwear to keep warm.  This compensates for the loss of extra heat normally supplied by customary muscle mass of young adulthood.

Pictures depict loss of muscle mass 

The first illustrates the extent of muscle loss that typifies sarcopenia.  The second picture show actual data of  the cross-section of the thigh region obtained from MRI scans of 3 volunteers:  a 40 year old triathlete, a 74 year old sedentary man and a 70 year old triathete.  Triathlete are athletes who compete in the triathlon (competitive biking, running, swimming events).  In each cross-section of the thigh muscle, the small white center circle is the bone.  It is surrounded by dark material (muscle) and defined by an outer sheath.  Thigh scans of the 40 and 70 year old triathlete are remarkably similar.  However, major changes are observed with the center photo of a 74 year old sedentary man.  Muscle tissue has disappeared and the space formerly occupied by muscle cells has been replaced with adipose tissue, another name for fat.  This sobering image emphasizes the actual extent to which skeletal muscle can disappear.  Regrettably, as serious as muscle mass disappearance may appear, the associated reduction in strength is several fold greater than the observed loss of mass!

Significant loss of muscle mass

It seems reasonable to assume that if one understands the severe consequences of aging in skeletal muscle, then what must follow is both an interest and a motivation to avoid them with proven interventions.  My next blog (Insight 3) will discuss strategies that achieve this goal.